Fig. 3
From: Microenvironments on individual sand grains enhance nitrogen loss in coastal sediments
The development of anoxic microenvironments and associated N-loss on a modelled sand grain colonized with O2 consuming and producing microbial colonies. (A) Patchy distribution of oxygen (O2) producing and consuming microenvironments within the diffusive boundary layer (DBL) indicate the fast cycling of O2 on micrometer scales and the formation of anoxic microenvironments on the sand surface, despite the presence of bulk ambient O2. (B) Even though zones of production and consumption are in direct proximity, anoxic microenvironments are formed in the presence of O2 resulting in the production of nitrogen (N2) through denitrification. Streamlines indicate the flow field (scale bar in (A), (B) represents 200 μm). (C) Phase diagram indicating the anoxic volume (in which rates are taking place) in dependence to pore water velocities and inflow O2 concentrations (based on 80 model runs indicated through gray dots, white dot indicates model run depicted in (A), (B)). At lower flow velocities (below 10 μm s−1), the volume of anoxic microenvironments is strongly dependent on the flow velocities through which the boundary layer thickness (diffusive length scale) is determined. At higher flow velocities (above 100 μm s−1) the anoxic volumes mainly depend on inflow O2 concentrations. An additional 80 model runs were carried out where no O2 production was occurring to simulate a scenario with no photosynthetic activity, and showed similar results (Fig. S9 and supporting information text).
